First, open the .fla file in Flash, and locate the item you want to remove.
Check in the Layers list (left column of the Timeline) to make sure all Layers are unlocked. The padlock icon is displayed when a Layer is locked - click it to unlock the Layer.
Then, click once on the item you wish to remove on the stage to determine what Layer it is on. Then, with that Layer selected (its color will be black, as shown in Fig. 2 above), click once on the TrashCan icon. This will delete the Layer and remove the item.
If the item you want to remove is not in a Layer by itself, and you wish to keep whatever else is in the Layer, then you may click once on the unwanted item on the stage to select it, and hit the Delete key on your keyboard.
Note: Most items deleted in this manner will remain in your Flash movie's Library, should you ever wish to re-insert them into your movie.